1. Field of the Invention
This invention relates generally to inflatable wellbore packers, and specifically to external casing packers as well as a method of casing an openhole wellbore.
2. Description of the Prior Art:
External casing packers are wellbore packing devices which are coupled into a string of casing. Preferably, the external casing packer includes a mandrel which defines a central bore which is substantially similar in internal diameter to that of the central bore of the casing string. A packing element is disposed radially outward from the mandrel, and serves to grippingly and sealingly engage a wellbore surface, such as an openhole wellbore wall.
External casing packers are used in conjunction with casing cement as a means for securing the casing string in a desired position within the wellbore, but can also be used in lieu of cement in certain applications. In such applications, the external casing packer is intended to remain in an inflated setting position for the useful life of the oil and gas well, which can be substantial periods of time.
One type of external casing packer includes an annular inflatable wall disposed about the mandrel of the external casing packer, which in-part defines an inflation chamber. Pressurized wellbore fluid is directed into the inflation chamber, which serves to receive pressurized fluid which outwardly radially expands the annular inflatable wall from an uninflated running mode of operation to an inflated setting mode of operation.
The prior art external casing packers are susceptible to two problems, each of which could result in catastrophic loss within the wellbore. The first problem is that the annular inflatable wall which is radially expanded outward in response to pressurized wellbore fluid is usually at least in-part composed of rubber. Typically, the annular inflatable wall includes an inner annular elastomeric sleeve which is covered on its exterior surface by protective material to prevent puncture of the elastomeric sleeve. After setting, the material which comprises the elastomeric sleeve is susceptible to "cold flowing". This could cause a change in pressure exerted against the annular inflatable wall, which could cause the external casing packer to release from gripping and sealing engagement with the wellbore wall, resulting in shifting of the casing string within the openhole wellbore or creation of a leak path around the packer. The second problem is that the inflation chamber of the external casing packer may include tiny leak paths which, over time, result in a loss of pressure from the inflation chamber, and corresponding loss of sealing engagement between the external casing packer and the openhole wellbore wall, also resulting in shifting of the casing string or creation of a leak path around the packer.